Compositions containing hexavalent chromium, sodium or potassium ferricyanide, and fluoride ion in acidified aqueous solutions have been used for producing corrosion-resistant conversion coatings on aluminum and its alloys. Representative of such compositions are those in U.S. Pat. No. 2,796,370 (patented June 18 1957). I have discovered that by substituting sodium nitroferricyanide(or sodium nitroprusside) for the sodium or potassium ferricyanide, the build-up of the deposit resulting from the reaction between the active components of the solution with the surface of the aluminum and its alloys was much more controlled, substantially eliminating powdering and the formation of porous, loosely-adherent coatings. Additionally, the appearance of the coatings produced by the sodium nitroferricyanide solutions unexpectedly improved in color from a matte, reddish-brown to a semi-glossy, golden yellow.